Lining of Irrigation Canals

Most of the canals, constructed in India to carry irrigation water, areunlined, and hence, a large part of the costly irrigation water is lost inpercolation and absorption as seepage loss.

No doubt, there are regions where the soil is such that seepage lossesare very small, and there is no justification for lining them, but at thesame time, it is also true that there are areas where 25 to 50% of thewater is lost in seepage.

This is a very serious loss and proportionately reduces the irrigationpotential of the supplied water.

Such seepage loss of the costly irrigation water must, therefore, beminimised. The seepage can be avoided or minimised by lining theirrigation canals.

Advantages of Lining

1. Seepage Control:

A lined canal costs about 2 to 2.5 times as much as an unlined canal,but where seepage is heavy, the saving of costly irrigation water mayitself be sufficient to fully justify the capital expenditure on lining.

It should also be kept in mind that heavy seepage losses in canalswould necessitate the construction of larger reservoirs and biggerdams.

Prevention of seepage by lining would, therefore, reduce theirimpounding capacity, and hence, lower the construction costs ofthese works

2. Prevention of Water-logging

Uncontrolled seepage through unlined canals, often raises the water-table in the surrounding fields up to or near to the ground level, as tobring the crop (plant) roots within the capillary fringe. This in turn,brings up the alkali salts near to the ground surface, which soonrenders the land unfit for cultivation. Such a land is usually called thur,and the phenomenon of rise of water table is known as water-logging.

Lining of canals prevents seepage and, thus, protects cultivable land.Combined with land drainage schemes, lining helps to reclaim water-logged areas.

3. Increase in Channel Capacity

The capacity of a chosen canal section can be considerably increasedby lining it.

The lining presents a smooth surface and, therefore, causes lessresistance to the flow of water. The water, therefore, flows faster, andhence, more of it is carried per second than that in an unlined canal.

Lining increases channel capacity and consequently reduces therequired channel section. Hence, for a new designed project, a linedchannel will require lesser dimensions and hence, lesser earth work.

The consequent saving in earth work handling (i.e. excavation andfilling) and acquisition of land, thus, become possible by canal lining.

4. Increase in Commanded Area

A lined canal can be designed not only smaller in cross-section but also shorter in length. The steeper gradients can be provided because higher velocities are permissible (as the material is less erodible) and a shorter alignment can, therefore, be selected.

On the other hand, flatter- slopes can be provided without silting on a lined channel compared to these on an unlined channel. It can, therefore, help to bring high areas under command.

5. Reduction in Maintenance Costs

This expenditure may be required on :

I. Removal of silt

A lined canal is not susceptible to erosion. It is usually designed tocarry the sediment load likely to enter at the canal headworks.

Moreover, on account of the high velocities in lined channels, thesand blown into it during sand storms, which may occur duringsummer in areas like Rajasthan deserts, etc. is readily carried away.

This eliminates or considerably reduces the annual expenditurerequired on unlined channels for desilting.

ii. Minor repairs

Periodical plugging of holes burrowed by rats, insects, etc., isconstantly required in unlined channels, failing which, breaches ofchannel embankments may occur.

The provision of adequate lining, reduces the danger of thesebreaches, and lesser vigilance is required.

iii. Removal of weeds

Huge money is spent in removing weeds and water plants likehyaocinth, etc., from the canals.

Lining eliminates or reduces the expenditure considerably, as theplants flow down the canal due to higher velocites in lined canals.

6. Elimination of Flood Dangers

An unlined canal founded on weaker foundations is always in danger,and a breach may occur at any time. Instances have occurred wheresmall breaches in unlined canals resulted in washing away ofconsiderable length of embankmentleading to flooding of certainareas and causing scarcity of irrigation water in others, as the canalwas out of service at a critical time for crops. A strong concrete liningremoves all such dangers.

DESIGN OF LINED IRRIGATION CHANNELS

Irrigation canals should be aligned and laid out, so that the velocity offlow is uniform under all conditions, and so that the water reaches theirrigated area at an elevation sufficient to ensure even and economicaldistribution.

High velocities of flow can be permitted by taking the advantage ofhard wearing surface, so as to ensure a hydraulically efficient channel.

Very high flow velocities, even if not damaging to lining, do entail extraexpenditure at turnouts, and require higher walls to take care ofpulsations or wave action.

While aligning the channel, sharp curves should also be avoided, asthey not only reduce the velocity of flow, but also require higher wallson the outside to retain the water as it rounds the curve.

Channel Cross-sections

Generally, two types of channel sections are adopted,

(i) Triangular channel section for smaller discharges,

(ii) Trapezoidal channel section for larger discharges.

In order to increase A/P ratio, the comers are rounded and attemptsare made to use deeper sections by limiting depth, etc. The sectionsand their properties, most commonly used.

Triangular Section

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Trapezoidal Section

Permissible Velocities in Lined Channels

Higher velocities can be safely used in lined channels. Thoughmaximum permissible velocities for concrete linings have not beenestablished, velocities up to 2.5 m/sec. (8 ft./sec) are permitted whenthe lining is not reinforced.

The concrete lined Nangal Hydel Channel with a capacity of 355cumecs is designed for a permissible average velocity of 1.8 m/sec. (6ft./sec.). If still higher velocities are desired, the lining can bereinforced accordingly. Asphaltic concrete, which has less resistance toabrasion, can withstand maximum velocity up to 1.5 m/sec. (5 ft./sec.),

Type of lining Permissible Velocity

Cement concrete lining (Unreinforced) 2 to 2.5 m/sec

Burnt clay tile lining 1.8 m/sec

Boulder lining 1.5 m/sec

Free Board in Lined Canals

Distance measured above the full supply level and up to the top of lining is known as the free-board for lined canals.

The minimum values of free-board for lined canals are specified by BIS code IS : 10430-1982

(A) Hard Surface Linings

(1) Cast insitu Cement Concrete lining

(2) Shotcrete or Plaster lining

(3) Cement Concrete tile lining or Brick lining

(4) Asphaltic Concrete lining

(5) Boulder lining

(B) Earth Type Linings

(1) Compacted earth lining

(2) Soil cement lining

TYPES OF LININGS

Hard Surface or Rigid Linings

Cast Insitu Cement Concrete Lining

Cement concrete lining made from M15 cement concrete mix (1:2:4) isconsidered a good quality type of lining.

Such linings usually give very satisfactory service, and are widely usedowing to their durability, impermeability, hydraulic efficiency, and forproviding weed free surface.

Despite the fact that the initial cost of C.C. lining is high, yet its long life andminimum maintenance cost usually makes it an economical type of liningover the life span. For this reason, concrete linings have been extensivelyused in America and other developed nations.

Concrete linings have been used in India in the canals of various importantprojects, such as Bhakra-Nangal Project, Tun- gabadhra Project, AmaravatiProject, Sarda Canal in U.P., etc. About 570 km length of channels havebeen lined in concrete in Bhakra-Nangal Project.

Sub-grade preparations

Cement concrete linings are best suited for main canals which have to carry huge flows and at higher velocities. However, being rigid, its success depends to a great extent upon a stable foundation.

Particular care should, therefore, be taken on thorough compaction of side slopes of the canals, on which the lining is to be laid.

Natural earth in cutting, is usually, satisfactory ; but embankments in filling must be compacted by some suitable means.

Ordinarily, any rock or earth or predominantly sandy soil, is suitable as a subgrade material for cement concrete canal linings.

Thickness

A thickness of about 5 to 15 cm of cement concrete is generally used forlarger canals, depending upon the canal capacity, the nature of canal,and the special requirements of imperviousness and structural strengthto resist cracking, on slight movement of subgrade. Minimum thicknessused in India is 7.5 cm. Stable side slopes of the order of 1.5 : 1 or1.25 : 1 should be adopted.

Steeper slopes entail extra earth pressure and consequently requiremore thickness, and hence are uneconomical.

Laying of Concrete

The levelled and dressed sub-grade soil surface shall be moistenedthoroughly before laying cement concrete, so that the moisture is notwithdrawn by the sub grade from the cement concrete.

Finishing

The surface of concrete finished against forms shall besmooth and free from projections, honey-coming and otherobjectionable defects.

The chipped openings shall be sharp and shall not be lessthan 70 mm in depth.

Curing

Subsequent to laying of concrete lining and after a period of 24-36hours, the lining shall be cured for at least 28 days.

On bed, this may be done by constructing 150 mm deep earthenbunds across the bed, so that a small depth of water will regularlystand on the bed.

The curing of side slopes may be done by constructing masonry drainswith weep holes or perforated pipes on the coping at the top oflining, or by sprinklers.

Surface drainage

Concurrently with the curing operation, surface drainagearrangement of the bank such as construction of dowels, banksurface slope - away from the lining, and construction of longitudinaldrain on the outer wedge shall be completed.

This is necessary to prevent surface and subgrade erosion andconsequent damage to the liming.

Joints in cement concrete lining

Cracks in C.C. lining usually occur due to :

I. Warping stresses caused due to the difference in temperaturebetween the atmosphere and the concrete lining, or due tomoisture potential between the two faces of the C.C. slab ; and

II. Tensile stresses caused by the differential temperature variationbetween the upper and lower faces of the C.C. slab.

Cracking caused due to above causes is controlled either by providingreinforcement or by providing joints in concrete lining.

The various types of joints which may be provided in C.C. lining are :

I. Expansion joints ;

II. Construction joints ; and

III. Contraction joints

Expansion joints are usually not provided in C.C. lining, except wherestructures intersect the canal. At the intersecting structure, an expansionjoint of 25 mm width, filled with sealing compound, is provided.

Construction joints which are left during casting of cement concretelining as pointed out earlier, do serve the purpose of contraction joints,which are specifically required to take care of shrinkage and temperaturestresses. Each construction joint will oppose contraction stresses, andhence will he a contraction joint.

Reinforcement

It has now been widely accepted that normal steel reinforcement(0.25 to 0.3% of concrete) adds practically nothing to the structuralstrength of uncracked lining.

But it has been found that reinforcement reduces the width ofshrinkage cracks, thereby reducing seepage and prevents possiblefaulting of the cracked slabs where unstable subgrades areencountered.

In long slabs of more than 15 m or so, the intervals betweentransverse cracks was found to be more in unreinforced slabs ascompared to those in reinforced slabs.

Advantages of cast in situ cement concrete linings

(i) Longer life than that of any other type.

(ii) Least permeable of all types.

(iii) Most resistant to erosion.

(iv) Permits fast construction by mechanical means.

(v) Low recurring maintenance charges.

Disadvantages of cast-in-situ cement concrete linings

(i) Higher initial cost.

(ii) Greater possibility of temperature cracking.

(iii) Less flexible and easily affected by adverse subgrade conditions.

(iv) Skilled supervision and construction necessary.

Advantages of pre-cast cement concrete tile linings

I. Higher strength for equivalent thickness

II. Avoids plaster finish

III. No lead of raw materials.

IV. Lesser skilled labour, as campared to that required for cast in situ c.c. lining, is required for pre-cast c.c. tile lining.

V. Easy to repair.

VI. Various types of joints possible,

VII. Lesser form work than what is reqd for cast in situ c.c. lining, is required here.

Disadvantages of pre-cast cement concrete tile linings

I. Slow progress,

II. Not suitable for curves.

III. Too light for hydel channels.

Shotcrete Lining Shotcrete is a technical term used to designate cement mortar applied

under pressure through a nozzle on the surface of the channel.

It consists of a mixture of cement and sand (generally in the ratio of 1:4).Sand is having a maximum size of 0.5 cm. Larger proportions of cementare required in shotcrete as compared to what is required in cementconcrete.

Wire mesh reinforcement is generally, although not necessarily, used inshotcrete canal linings.

It is also useful for repair works and in rehabilitation of old canals.

Shotcrete linings can be placed in an irregular canal section, thuseliminating the necessity of trimming the section, as is required forconcrete canal linings when placed with a slip-form.

Cement Concrete Tile Lining or Brick Lining Such types of linings are very popular in India, because of certain

inherent advantages in their use.

(i) Bricks or concrete tiles can be laid by ordinary masons, and speciallyskilled labour, as reqd for cast insitu c.c. lining, is not required.

(ii) Rigid quality control is not required.

(iii) No expansion joints are required.

(iv) Rounded sections can be easily laid without using any formwork.

(v) Larger number of labour is required, thus providing greateremployment potential.

(vi) Isolated damaged portions can be repaired easily.

(vii)Bricks can be plastered to increase the carrying capacity of canal withthe same section, and also to help increase the life span of lining.

Asphaltic Concrete Lining

Asphalt has been used as a lining material at a very few places. It is stillunder the stage of evolution.

Asphaltic concrete is a carefully controlled mixture of asphalt andgraded stone aggregate, mixed and placed under elevated temperature.

It provides a fairly cheap lining, especially where the asphalt is availablewithin the country. It is flexible and readily confirms to the subgrade.

The disadvantages or the limitations of this type of lining are :

(i) It does not decrease the rugosity coefficient of the channel.

(ii) It permits certain type of weed growth.

Boulder Lining Boulder lining, also called dry stone lining or stone pitching, consists of lining

the side slopes of an earthen canal by proper placement and packing of stones,either after laying a filter layer over the soil surface, or without any such filter,depending upon the site requirement.

Such a lining does not prevent seepage of canal water, though helps inretaining the shape of canal section, thereby reducing maintenance cost.

The stones to be used for lining are rounded or sub-angular river cobbles, orblasted rock pieces with sufficient base area, so as to remain stable in theirposition.

The biggest advantage of such a lining however, is that it is a pervious liningallowing free flow of water from the submerged or saturated subgrade into thecanal.

Such a lining, therefore, does not need any drainage arrangements, in the formof pressure relief valves, etc. as may be required in concrete or brick linings.

Earth Type Linings

Compacted Earth Lining

Soil graded to obtain the required characteristics and containingenough fines, so as to make it impervious, is thoroughly compacted atoptimum moisture content, and is used to provide a lining of 30 to 90cm thickness.

The use of this type of lining is restricted to the availability of suitablesoils in the area, through which the canal is being constructed.

It would be uneconomical to transport the selected soils fromoutside. Special drainage arrangements are provided for this.

Soil-Cement Lining

Portland cement up to the extent of 2 to 8% is added to the soil having a high percentage of fines.

The mixture is first mixed dry. Water is then added so as to bring the soil to its optimum moisture content, and again mixed thoroughly.

Material is then placed at site and compacted. Curing is then required for atleast seven days by covering with wet sand, etc.

Requirements of Good Lining

(1) Economy;

(2) Structural stability ;

(3) Durability ; and

(4) Repairability

Additional requirements

1. Impermeability;

2. Hydraulic efficiency; and

3. Resistance to erosion

Factors Responsible for Selection of a Particular Type of Lining

1. Size and Importance of the canal

2. Canal Slopes and Alignments

3. Climate of the Area

4. Availability of Materials

5. Initial Expenditure

Under Drainage of Lined Canals